1. Field of the Invention
The invention relates to a light source device, especially to a light source device which is used for the optical system of a projector device using liquid crystals and a DMD device (D(igital) M(icro mirror) D(evice)).
2. Description of the Prior Art
Recently, there has been a great demand for prolonging the service life of a projector device and making it smaller. There is also a demand for increasing the radiance of the discharge lamp as the light source. As the light source, a discharge lamp is suggested in which, by an increased mercury vapor pressure, the broadening of the arc is suppressed (the arc is compressed) and a distinct increase of radiance is enabled. This discharge lamp is disclosed, for example, in JP-A-2-148561 (corresponding to U.S. Pat. No. 5,109,181) and JP-A-6-52830 (corresponding to U.S. Pat. No. 5,497,049).
However, if the mercury density within the arc tube is increased, there are cases in which due to the occurrence of a small temperature change within the arc tube unstable behavior repeatedly occurs, in which the added mercury in the area with a low temperature does not vaporize to some extent and afterwards the unvaporized mercury vaporizes. The repetition of this unstable behavior makes the emission of the discharge lamp unstable. In particular, due to the sudden vaporization of the unvaporized mercury, the emission of the discharge lamp instantaneously becomes unstable, by which the so-called flicker phenomenon occurs in a projector device.
On the other hand, in the operation of a projector device, in addition to use in operation with normal radiance (steady-state operating mode), there is also a demand for power-saving operation (economy mode) with reduced operating wattage of the discharge lamp. Specifically, there is a case in which it is preferred that the projected images of the projector device be viewed as darker images, and a case in which it is preferred that the projector device be used with a lower noise burden with reduced rpm of the air cooling fan. Since this case is accompanied by a reduction of the operating wattage of the discharge lamp, the formation of the above described unvaporized mercury is further accelerated as a result.
Even if the lamp can be built such that the mercury can completely vaporize in the economy mode a problem still would exist. That is, in the steady-state operating mode, excessive increases in the temperature of the lamp occurs, resulting in the disadvantages of devitrification of the discharge vessel and the like.
Furthermore, if the device can be built such that, by arrangement of a cooling device in which the cooling capacity can be regulated within a wide range (for example, by the arrangement of a cooling fan), in the economy mode, the mercury is caused to completely vaporize and that, in the steady-state operating mode, an excess temperature increase is avoided, the provision of the fan would itself be problematic. That is, since in a projector device there is a demand for making it smaller (for example, B5 size) and lighter in weight (for example, roughly 1500 g), the provision of such a large cooling device is not practical.
Therefore, even in a projector device provided with an economy mode, in practice, the operating wattage cannot be regulated in a wide range. The genuine state of affairs is that, for example, in a discharge lamp with a rated output of 200 W, a wattage reduction to at most roughly 80% (operating wattage of 160 W) can be accomplished.
The same disadvantage can occur in the case in which the wattage is changed not only in the classification of the steady-state operating mode and the economy mode, but also in the steady-state operating mode.